Sediment Flow Paths and Associated Organic Carbon Dynamics Across A

Home , Ait

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ doi:10.5194/hess-19-1209-2015 © Author(s) 2015. CC Attribution 3.0 License.

Sediment ﬂow paths and associated organic carbon dynamics across a Mediterranean catchment

C. Boix-Fayos1, E. Nadeu1, J. M. Quiñonero2, M. Martínez-Mena1, M. Almagro1, and J. de Vente1 1Soil Erosion and Conservation Research Group, CEBAS-CSIC, Spanish Research Council, Campus de Espinardo 30100, P.O. Box 164, Murcia, Spain 2Department of Geography, University of Murcia, Campus de la Merced, 30001 Murcia, Spain

Correspondence to: C. Boix-Fayos ([email protected])

Received: 28 February 2014 – Published in Hydrol. Earth Syst. Sci. Discuss.: 15 May 2014 Revised: 21 November 2014 – Accepted: 4 February 2015 – Published: 3 March 2015

Abstract. Terrestrial sedimentation buries large amounts of the relatively short transport distances, the effective preserva- organic carbon (OC) annually, contributing to the terres- tion of OC in microaggregates and the burial of sediments in trial carbon sink. The temporal significance of this sink will the alluvial wedges gave rise to low OC mineralisation, as strongly depend on the attributes of the depositional envi- is arguably indicated by C : N ratios similar to those in soils. ronment, but also on the characteristics of the OC reach- Deposits in middle stream areas (fluvial bars) were enriched ing these sites and its stability upon deposition. The goal in sand, selected upon deposition and had low OC concen- of this study was to characterise the OC during trans- trations. Downstream, sediment transported over longer dis- port and stored in the depositional settings of a medium- tances was more selected, poorly microaggregated, and with sized catchment (111 km2) in SE Spain, to better understand a prevalence of silt and clay fractions and MOC pool. Over- how soil erosion and sediment transport processes deter- all, the study shows that OC redistribution in the studied mine catchment-scale OC redistribution. Total organic car- catchment is highly complex, and that the results obtained bon (TOC), mineral-associated organic carbon (MOC), par- at finer scales cannot be extrapolated at catchment scale. Se- ticulate organic carbon (POC), total nitrogen (N) and par- lectivity of particles during detachment and transport, and ticle size distributions were determined for soils (i), sus- protection of OC during transport and deposition are key for pended sediments (ii) and sediments stored in a variety of the concentration and quality of OC found at different de- sinks such as sediment wedges behind check dams (iii), positional settings. Hence, eco-geomorphological processes channel bars (iv), a small delta in the conjunction of the during the different phases of the erosion cycle have impor- channel and a reservoir downstream (v), and the reservoir tant consequences for the temporal stability and preservation at the outlet of the catchment (vi). The data show that of the buried OC and in turn for the OC budget. the OC content of sediments was approximately half of that in soils (9.42 ± 9.01 g kg−1 versus 20.45 ± 7.71 g kg−1, respectively) with important variation between sediment deposits. Selectivity of mineral and organic material during 1 Introduction transport and deposition increased in a downstream direction. The mineralisation, burial or in situ incorporation of OC in Terrestrial ecosystems have captured up to 28 % −1 deposited sediments depended on their transport processes (2.6 ± 0.8 Pg yr ) of the CO2 emitted annually over and on their post-sedimentary conditions. Upstream sedi- the last decade (Le Quéré et al., 2013). Among the pro- ments (alluvial wedges) showed low OC contents because cesses involved in this terrestrial carbon (C) sink, terrestrial they were partially mobilised by non-selective erosion pro- sedimentation of eroded soil and replacement of soil organic cesses affecting deeper soil layers and with low selectivity of carbon at eroding sites have been regarded as active com- grain sizes (e.g. gully and bank erosion). We hypothesise that ponents (Stallard, 1998; Harden et al., 1999; Van Oost et al., 2007). The magnitude of its contribution to the sink has

Published by Copernicus Publications on behalf of the European Geosciences Union. 1210 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics been estimated by some to be between 0.6 and 1.5 Pg of C 2013; Nadeu et al., 2011, 2015). The study of OC transport annually (Stallard, 1998; Aufdenkampe et al., 2011) through from erosion sites to depositional settings implies the con- the burial of large quantities of laterally transported organic sideration of a large number of factors and processes taking carbon (OC). The significance of this contribution, however, place. Several studies have described the impact of variation will depend on the long-term preservation of the buried OC, of transport and deposition of different OC size fractions and an issue that remains under debate (Van Oost et al., 2012 the role of OC mineralisation, as well as the breakdown of and references therein). The fate of the redistributed OC soil aggregates or re-aggregation at depositional sites (Wang will ultimately depend on the mechanisms of its physical et al., 2010; Van Hemelryck et al., 2011; Martínez-Mena et and chemical protection against decomposition, its turnover al., 2012), and the contribution of new OC formation from rates and the conditions under which the OC is stored in vegetation at depositional settings. Altogether, these factors sedimentary settings (Van Hemelryck et al., 2011; Berhe and are considered responsible for the transformations undergone Kleber, 2013). by OC from source to sink. Yet, comprehensive studies of The study of the temporal evolution of buried OC at de- source to sink processes are, to the best of our knowledge, positional sites can be approached from different and com- lacking. plementary perspectives. It has been observed that organic The objective of this study is to characterise the OC in matter exported from rivers into the sea is not necessarily transit and at a range of depositional settings (Table 1) in a identical to the organic matter of the plants and soils up- medium size catchment (Fig. 1) and to associate our obser- stream in the river catchments (Raymond and Bauer, 2001). vations with the catchment sediment dynamics. We aimed to This indicates that tracing sediment from source areas and (i) characterise the OC concentrations in the main sedimen- the processes taking place during transport and deposition tary deposits along the catchment’s drainage system, (ii) as- of eroded OC can also provide information on the quality sess the main processes involved in sediment redistribution, and dynamics of the eroded OC (Nadeu et al., 2012 and ref- and (iii) establish links between these processes and the OC erences therein). Actually, more than 90 % of the sediment concentration and quality. generated annually in uplands is not exported from catchments (Trimble, 1983; Meade and Stevens Jr., 1990; Walling and Fang, 2003) but remains in transitory depositional sites 2 Study area such as lakes and reservoirs, colluvial deposits at the bases of hillslopes, alluvium in floodplains and channel bars (Meade The study area is located in the headwaters of the Segura and Stevens Jr., 1990). In fact, flood plains are expected to catchment (Murcia, SE Spain), which drains to the Taibilla represent a key storage site for OC within the catchment C reservoir (Turrilla catchment) and is formed by three adjacent balance and, increasingly, this function is fulfilled by reser- subcatchments (Rogativa, Arroyo Blanco and Arroyo Ter- voirs (Verstraeten et al., 2006; Wisser et al., 2013; Ran et cero) covering a total area of ∼ 111 km2 (Fig. 1). The Taibilla al., 2014). Although large efforts are being made to under- reservoir, built in 1974, provides water to more than 2 million stand the flow paths of OC at the catchment scale, most of people. The dominant lithology of the catchment consists the abovementioned C sinks remain unquantified. Recently, of marls, limestones, marly limestones and sandstones of Ran et al. (2014) have estimated an OC budget for the Yellow the Cretaceous, Oligocene and Miocene. The mountains are River, concluding that over a period of 60 years, 49.5 % of mainly constituted by limestones, while the middle and bot- the OC was buried in different sinks within the river system, tom valley sections are dominated by marls (IGME, 1978). 27 % was mineralised during the erosion and transport phases The average annual rainfall for the period 1933–2004 was and 23.5 % was delivered into the ocean. However, there is 583 mm and the average annual temperature 13.3 ◦C at a sta- still large uncertainty over the stability and residence times tion located in the centre of the basin at 1200 m a.s.l. (above of OC in many of these sinks, which are affected greatly by sea level). Snow in the mountains, especially above 1700 geomorphological and hydrological dynamics (Hoffmann et m, is not abundant but is frequent in winter. The domi- al., 2013). Thus, characterising the OC at these transitory set- nant soils in the area are Lithosols, Regosols and Cambisols tings and acquiring knowledge on the processes and factors (IUSS Working Group WRB, 2006). They have an average that influence OC stability at these sites contribute to the as- OC concentration in the first 10 cm of between 3.2 and 1 % sessment of the significance of terrestrial deposition in the C depending on the land use, being the lowest for agricultural cycle. use mainly on marl lithology (Nadeu et al., 2014). A previ- Understanding how OC moves along with sediments ous study at the site showed that OC concentration in soil through the different phases and types of erosion and trans- profiles down to 1 m was 1.5 ± 1.4 % and 2.2 ± 1 % in pro- port processes is crucial to explain, partially, the large vari- files located in forest areas in two Rogativa subcatchments. ation in C contents found in depositional sites. Along these In both subcatchments average OC concentrations in channel lines, progressively the geomorphic factors that control the sediments down to 80 cm were 1.1 ± 0 % and 1.4 ± 0.1 %, redistribution of OC within watersheds are being defined respectively (Nadeu et al., 2012). Boix-Fayos et al. (2009) (Berhe and Kleber, 2013; Evans et al., 2013; Hoffmann et al., attributed variation of OC concentration in depth in sedi-

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1211

Figure 1. Location of the study area and sampling scheme. ment profiles down to 120 cm, located in the main channel the river bed and accelerating bank erosion processes (Boix- of seven subcatchments of Rogativa, to changes in the land Fayos et al., 2007). Land use changes have been estimated use pattern of the drainage area over the last decades. to be responsible for about 50 % of the reduction in catch- The landscape is a mix of dryland farming (mainly ment sediment yield (Boix-Fayos et al., 2008) and have had barley), plantations of walnuts (Juglans regia L.), forests an important impact on the soil carbon stocks of the catch- and shrublands. The forest is dominated by Pinus ni- ment (Boix-Fayos et al., 2009). gra Arn. subsp. salzmannii, although some individuals and The Turrilla catchment shows a dendritic channel pat- masses of Pinus pinaster Ait. and Pinus halepensis Mill. are tern. The main channel has an average slope of 7.7◦ and located in the lowest parts of the basin. A relevant propor- a total longitude of 22 km. The stream is discontinuous tion of the pine forest was planted in the reforestation works upstream and continuous downstream. Geomorphological associated with dam construction in the 1970s. Nowadays, characterisations of channel reaches along the main and masses of Quercus rotundifolia Lam. are isolated or associ- tributary streams of the Rogativa catchment indicated dom- ated with P. nigra subsp. salzmannii. inance of non-selective erosion processes affecting deeper The catchment has been affected by important land use soil layers and with no selectivity of grain sizes, such as changes since the second half of the twentieth century. These gully, bank and river bed erosion (Nadeu et al., 2012), often changes consisted mainly of a progressive abandonment of activated as a consequence of the decrease in sediment input the dryland farming activities and an increase of the forest from the adjacent slopes caused by a generalised recovery of cover. In the 1970s, a network of check dams was installed. the vegetation, following agricultural land abandonment and Previous studies highlighted the important impact of land reforestations (Nadeu et al., 2014). Furthermore, land use and use changes and check dams on the catchment’s sedimen- morphological characteristics of the drainage area were iden- tary dynamics (Boix-Fayos et al., 2008; Quiñonero-Rubio et tified to be important driving factors determining the concen- al., 2013, 2014), causing important morphological changes in tration and organic carbon yield exported by lateral fluxes www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 1212 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics

Table 1. Main location and characteristics of the sampling areas within the catchment.

Location of soil Group Drainage Deposition areas Profiles/ Maximum Samples and sediments name area (ha) subcatchments/ depth (n) events (cm) Soils Slopes Soils 11 000 109 10 109 Sediments Subcatchments Wedges 8–146 Sediment wedges 19 125 537 in the third- and behind check fourth-order dams channels Main channel Bars 5000 Channel bars 10 3–30 46 and a tributary stream Main channel Suspended 5000–7800 Suspended load 13 7–42 69 sediment Main channel Delta 11 000 Delta in the 6 10 24 conjunction of the main channel and reservoir Reservoir Reservoir 32 000 Reservoir 2 100 23 sediments at the exit of the catchment in smaller subcatchments of the Rogativa watershed (Nadeu samples, 20 % were from high-density forest soils, 30 % from et al., 2015). In general terms, the main channel of Roga- low-density forest soils, 20 % from shrubland soils, 10 % tiva moved from an aggradation period with large sediment from pasture soils and 20 % from agricultural soils. Dis- volumes coming from a well-connected agricultural catch- turbed samples were taken for laboratory analyses and undis- ment (1950s–1980s), to an incision and degradation phase turbed samples (rings of 100 cm3) for estimating soil bulk after afforestation, land abandonment and hydrological con- density. trol works (Boix-Fayos et al., 2007). Nowadays, the Rogativa catchment is under a transition phase with an armoured main 3.2 Sediment data channel and sediments are being incorporated in the channel through gullies and bank erosion (Boix-Fayos et al., 2007; 3.2.1 Alluvial wedges (behind check dams) Nadeu et al., 2011). The sediment deposited behind the network of check dams installed in the 1970s was used as representative of mate- 3 Methods rial mobilised by erosion processes and fluvial transport from the upper catchment areas (Figs. 1, 2). A total of 19 (sub) The field experimental design was based on the main path- catchments, evenly distributed, were sampled. The sediment ways of sediment and soil OC during their transport through wedges deposited behind each check dam were sampled at the catchment. Therefore, soils and different sedimentary de- the front (close to the check dam) to a maximum depth of posits within the catchment were sampled as shown in Ta- 1.25 m. In addition, 14 of the sediment wedges were sam- ble 1 and Fig. 2. pled also at the back of the sediment wedge, to a maximum depth of 96 cm. At all points, bulk samples of 100 cm3 were 3.1 Soil data taken at intervals of 7 cm depth until the maximum depth was reached. Moreover, for 14 wedges, replicate samples of the Topsoil (0–10 cm) samples were taken from 109 locations first layers were taken at 7 cm intervals, to 35 cm depth. A distributed from upstream to downstream in the catchment, total of 537 undisturbed samples were collected. representing all land uses and their spatial extent. Of these

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1213

Figure 2. Sketch representing the morphological positions selected for sampling of soil and sediments. Soils were sampled all around the catchment in all the different land use classes, alluvial wedges behind check dams predominantly in the upper and middle parts of the catchment, while alluvial bars and suspended solids were sampled in the middle and downstream areas. Delta and reservoir represent sampling areas downstream.

3.2.2 Fluvial bars 3.2.3 Suspended sediment

Sedimentary fluvial bars located at two different reaches in Two devices for the sampling of suspended sediments were the Rogativa main stream, 2 km apart in the middle sec- installed in the main stream of the Rogativa channel in Oc- tion of the catchment, were sampled. One of the reaches tober 2010: one was installed in the downstream area, drain- is a permanent-flow reach where four different bars were ing a catchment area of 54.4 km2, and the other was installed sampled during three seasons (autumn 2009, winter and below the confluence with the perennial stream of Arroyo spring 2010), and the other is an intermittent reach where Blanco, draining a total catchment area of 78.1 km2. four bars were sampled in autumn 2009 and winter 2010 (the The sampling devices consisted of a column of six bottles same dates as the bars of the permanent-flow reach). A dis- located at an averaged height difference of 7.5 cm from each turbed sample of the first 5 cm of the bar was taken at each other (Figs. 1, 2). At both locations the upper bottle repre- sampling period, as no different layers corresponding to dif- sented bank-full conditions of the incised stream. A total of ferent events could be distinguished in these bars. 69 samples, corresponding to 13 events over a 2.5 year period Two more bars, also in an intermittent reach in the middle were collected. section of the catchment and in a confluence of a small sub- catchment (11 ha, Barranco Escalerica 2) and the Rogativa 3.2.4 Delta sediments main stream, were sampled. The bars in the stream bed were incised during the last runoff event (winter 2010) that took Sediments in the confluence of the river channel and the place a few days before the sampling. The deposited layers reservoir were sampled. This delta area is characterised by corresponding to different runoff events could be identified a very gentle slope and point bars formed by the meandering (Figs. 1, 2) and were sampled to a maximum depth of 30 cm contact between the river and the reservoir. A sampling (bar 1, seven layers, average depth of each layer 4.2 cm) or scheme following the convex depositional areas of the me- 20 cm (bar 2, six layers, average depth of each layer 3.3 cm). anders of the river towards the reservoir was implemented: a These bars represented a mixture of sediments coming from total of six positions with two replicates and two depths (0–5, several erosion processes and were considered representative 5–10 cm) were sampled, collecting a total of 24 samples. of the type of sediment being transported along the stream bed from further upstream (Nadeu et al., 2011). A total of 3.2.5 Reservoir sediments 46 samples were collected. Sediments in the Taibilla reservoir were sampled in March 2010. Water height in the Taibilla reservoir is highly variable between years and during the year. Samples were www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 1214 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics taken at a distance of 500 m from the confluence of the main 3.4 Statistical analysis stream of Rogativa and the reservoir, in exposed sediments forming a terrace 20 cm above the water level at the moment Significant differences between averages were tested using of sampling. Sampling was done with a Cobra TT hydraulic the non-parametric Kruskal–Wallis (K–W) test at p < 0.05. hammer to a depth of 1 m. The 1 m deep core was divided at Spearman correlations were performed to explore the rela- 5 cm intervals to 20 cm depth and then at 10 cm intervals. A tionships between the TOC and pools, as well as C : N ra- total of 23 samples were analysed. tios and the percentages of primary and microaggregated soil particles, together with different aggregation indexes, for all 3.3 Laboratory analysis cases or each sediment reservoir type separately. All statistical analyses were performed with the software SPSS 19.0 All soil and sediment samples (from erosion deposits and de- (SPSS, Chicago, IL). position bars) were air-dried or dried in an oven at a low temperature (< 60◦) and then sieved at 2 mm. Primary particle size distribution was measured using a combination 4 Results of wet sieving (particles > 63 µm) and laser diffractometry (particles < 63 µm) using a Coulter LS, for the sand frac- 4.1 Primary particle size distribution of soils and tion and the silt and clay fractions, respectively. The organic sediments matter in these samples was oxidised with hydrogen per- oxide and chemically dispersed with a mixture of sodium The particle size distribution of soils differed from that of hexametaphosphate and sodium carbonate (anhydrous) for most of the sediments (Fig. 3). The alluvial bars showed, on 18–24 h. The fractions obtained were classified as coarse average, higher sand contents and lower contents in the fine sand (2000–250 µm), fine sand (250–63 µm), coarse silt (63– fractions than the soils (Fig. 3). This enrichment in coarse 20 µm), fine silt (20–2 µm) and clay (< 2 µm). The effective fractions in the alluvial bars was accompanied by the lowest particle size distribution was measured by introducing air- TOC (Fig. 4a). In contrast, the suspended solids and the delta dried samples into a Coulter LS, without previous physical and reservoir sediments had higher percentages of silt and or chemical dispersion. From these two measurements, mi- clay than the soils. The high sand contents in the reservoir croaggregation indices were derived. and delta sediments were similar to the sand content (around The percentage of microaggregated particles of fine sand, 20 %) of the wedges. The particle size distributions of the coarse silt and fine silt sizes and two aggregation indices that alluvial wedges were more similar to those of the soils of the give an indication of the total percentage of microaggregated catchment than to those of the rest of the deposits (Fig. 3). particles were used: IA (index of aggregation), as the sum of the differences of the dispersed and non-dispersed material in 4.2 Microaggregated particles in soils and sediments each size group (Wang et al., 2010), and ASC (aggregation Based on the microaggregation data, two groups can be dis- silt and clay) as the difference between dispersed clay and tinguished. Soils and sediment wedges with similar IA and silt and non-dispersed clay and silt (Igwe, 2000). ASC values (Table 2) showed a significantly higher microag- Given that soil carbon pools might have differences in gregation level than suspended sediment and reservoir sedi- turnover times, it is of high interest to evaluate the behaviour ments. The sediments in alluvial wedges had 10 times more of different carbon pools separately, as this could give us large aggregates (250–63 µm) than the suspended sediment insight into the long-term stability of the mobilised carbon and 6 times more than the reservoir sediments. This was the by lateral fluxes. For this purpose the OC was divided into dominant class in soils and alluvial wedges, while no differ- physical fractions by wet sieving: particulate organic car- ences between size classes occurred in suspended and reser- bon (> 53 µm) (POC) was separated from mineral associ- voir sediments. The percentages of medium-sized microag- ated organic carbon (< 53 µm) (MOC) after shaking 10 g of gregates (63–20 µm) in the alluvial wedges were around 3 air-dried soil sieved at 2 mm with 50 mL of sodium hexam- and 7 times greater than in the suspended sediment and reser- etaphosphate for 18 h (Cambardella and Elliot, 1992). Frac- ◦ voir sediments, respectively. No significant differences were tions were oven-dried at 60 C for water evaporation and the found between the percentages of microaggregated particles dry material was weighed prior to OC determination. The OC in the suspended sediment and reservoir sediments, regarding and nitrogen contents were determined by dry combustion the total microaggregated material and size classes. in an elemental analyser (FLASH EA 1112 Series Thermo). The total organic carbon (TOC) was assumed to be the sum 4.3 Organic carbon and nitrogen in soils and sediments of the POC and MOC. The MOC accounted for microaggre- gate and intra-aggregate OC in the silt and clay size fractions. The TOC concentrations of sediments were, on average, Duplicate or triplicate soil samples were used for laboratory around half (9.42 ± 9.01 g kg−1) of the average TOC con- analysis. centrations of surface soils (0–10 cm) (20.45 ± 7.71 g kg−1), being higher in alluvial wedges behind dams, in the sus-

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1215

Figure 3. Median, percentiles (10th, 25th, 75th and 90th) and error of the main textural classes (A, sand; B, silt; C, clay) of the different types of sedimentary deposits within the Turrilla catchment. Different letters mean significant differences according to the Kruskal–Wallis test (p < 0.05). pended solids, in the delta and in the reservoir and lower 4.4 Correlations between primary and in alluvial bars in the main channel (4.44 ± 1.98 g kg−1) microaggregated particles, OC pools and C : N (Fig. 4). ratios The highest POC content was observed in sediment samples taken from the alluvial wedges, while suspended solids The Spearman correlation coefficients showed different and reservoir profiles contained the highest contents of patterns in the relationships between the percentages of pri- MOC. The lowest MOC content was found in the bars mary and microaggregated particles and the OC pools and (Fig. 4a and b). Similar POC : MOC ratios were found on C : N ratios across the deposit types (Table 3). The index of soils, wedges and bars (ranging from 0.5 in bars to 0.8 in aggregation (IA total amount of microaggregated particles) soils), whereas much lower POC : MOC ratios were found was positively correlated with TOC (for all data) and for for the delta and reservoir (∼ 0.2), reflecting the prevalence the suspended sediment (p < 0.06), POC (for all data and of the MOC pool transported downstream (Figs. 4c, 5b). alluvial wedges) and C : N ratio (of all data). Furthermore, Furthermore, while C : N ratios of the MOC and POC frac- the percentages of microaggregates of 250–63 µm were positions of soils sampled under different land uses in the catch- tively correlated with POC for all data and alluvial wedges ment did not show significant differences (data not shown, and negatively with the C : N ratio (for wedges and sus- K–W test, p > 0.05), most of the sediment deposits (except pended sediments). Percentages of microaggregates of 63– for alluvial wedges) showed significantly lower C : N ratios 20 µm were positively correlated to TOC and MOC of all than soils (Fig. 5a). In general, a decrease of C : N ratios of deposits data and of alluvial wedges. Furthermore this mi- sediment deposits along the fluvial path was accompanied croaggregate size was positively correlated to C : N of all by a simultaneous decrease in N, with the exception of sus- data, soils and wedges. The smallest microaggregated parti- pended load, in which a higher N content was found than in cles (20–2 µm) were positively correlated to TOC and MOC the other deposits (data not shown, K–W test, p < 0.05). of the reservoir and to POC of all data. In contrast, the ASC index (microaggregated material of silt and clay size frac- www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 1216 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics

Figure 4. Median, percentiles (10th, 25th, 75th and 90th) and error of the total organic carbon concentration (TOC) (A), particulate organic carbon (POC) (B), and mineral associated organic carbon (MOC) (C) of soils and sediments within the Turrilla catchment. Different letters (a–e) indicate significant differences according to the Kruskal–Wallis test (p < 0.05). ∗ – the suspended solids measured contained only the MOC fraction. tions) was negatively correlated with the OC pools, only in posits (e.g. alluvial wedges and suspended sediments), indi- the cases where no correlation with fine-sized microaggre- cating selectivity during detachment and transport. However, gates (20–2 µm) was found. ASC correlated also negatively OC was also positively correlated with the content of sand with C : N ratios for all data, soils and alluvial wedges. In particles in the soils and reservoir, probably indicating the general, it seemed that the TOC content and its fractions cor- entrance of organic material from other processes (e.g. in situ related positively with microaggregated material (IA). Some formation of C in lakes; Tranvik et al., 2009) in the reservoir microaggregated sizes showed significant positive correla- and OC formation in the upper layers of sediment. Positive tions with OC of some deposits. Larger microaggregated correlations of the sand and OC in soils are probably due to sizes correlated with TOC and POC of alluvial wedges the presence of several samples of sandy soils covered by and the finest microaggregated size with TOC an MOC in dense forest (25 % of dense forest soil samples had 45–70 % the reservoir downstream. No consistent correlation patterns sand). were found between the percentages of primary particles and the OC pools and the C : N ratio across deposit types. Both total OC and the different pools were positively associated 5 Discussion with the sand fraction and negatively with the clay fraction when all the data were considered together, in soils (only sig- Hoffman et al. (2013) pointed out that accounting for the nificant association with sand) and in the reservoir. By con- non-steady state of C dynamics along flow paths from hill- trast, the clay fraction was associated positively with TOC slopes through river channels and into oceans is crucial to in wedges and suspended sediment, and with MOC in allu- understand the overall C budget. The redistribution of OC vial wedges. Overall, as may be expected, the OC content through soil erosion and sediment transport is determined was positively correlated with the clay fraction in some de- by processes that affect the mineral components of sedi-

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1217

Martínez-Mena et al., 2008). These relatively low OC concentrations found in sediments at the catchment scale raise the question: do the erosion and sediment transport processes lead to C losses to the atmosphere, or is there another explanation for this difference in C concentration between soils and sediments? Previous studies have also found a depletion of OC in sediments measured at a catchment scale (Avnimelech and McHenry, 1984; Haregeweyn et al., 2008; Chaplot and Poe- sen, 2012). For a small catchment (22 ha), Fiener et al. (2005) observed that sediment deposited in ponds was depleted in OC and clay relative to the source soils, while the sediment collected at the outlet of the pond was enriched in OC and clay compared to soils. This demonstrates the important changes in OC content that may occur during different transport and deposition phases due to preferential deposition. Similar results were reported by Wang et al. (2010) and Rho- ton et al. (2006), who found OC impoverishment in deposited sediments within their study catchments, while the sediment transported in suspension and exported out of the catchment was enriched in OC (1.2–3 times) compared to the OC concentration in the source soils. Ran et al. (2014) attributes also lower values of OC than soils for different sedimentary settings, based on several methods, to estimate the OC carbon budget of the Yellow River. In our case, the suspended sediments as well as the reservoir sediments showed significantly higher contents of MOC compared to the other sediment deposits, while the MOC contents in soils was similar to that Figure 5. Median, percentiles (10th, 25th, 75th and 90th) and error in reservoir sediments and slightly higher than in suspended of C : N ratios (A) and POC : MOC (B) ratios in soils and sediments sediments. This was also found by Amegashie et al. (2011) within the Turrilla catchment. Different letters (a–b) indicate signif- and can be probably attributed partially to new, in situ forma- icant differences according to the Kruskal–Wallis test (p < 0.05). tion of OC in the reservoir (Einsele et al., 2001). We suggest ND – no data available, ∗ – the suspended solids measured contained only the MOC fraction. that at these larger scales the decrease of OC in sediments compared to soils is due to a combination of factors related to the spatial scale of observation, namely (i) variety of sed- ment (e.g. selectivity and non-selectivity of material during iment sources, that dilute the source effect; (ii) interaction the detachment, transport and deposition phases) and by pro- of multiple erosion processes, both selective and unselective; cesses directly affecting the OC fraction within the sediments (iii) long transport distances that favour continuous remobil- (mineralisation, fixation, protection, new OC formation). To isation of sediment facilitating aggregate breakdown and a better understand the role of soil erosion and sediment trans- reduced physical protection of OC; and (iv) the ample time port in the overall C budget, based on the data reported in lapse (from hours to several years) that occurred between this study, the following paragraphs discuss the importance sediment detachment and its sampling. This contrasts with of spatial scale and particle size selectivity as well as the C sediments collected at the plot and hillslope scales, which dynamics of eroded sediments during detachment, transport are collected close to their sources during the detachment and deposition phases. and transport phases and often shortly after the erosion event, with little opportunity for OC mineralisation. 5.1 OC redistribution: effect of the scale of observation 5.2 Mechanisms of OC loss or gain in sediments All sediments studied within the Turrilla catchment showed, on average, OC concentrations (9.42 ± 9.01 g kg−1) lower Among all the above-mentioned factors and processes that than half of those of soils (20.45 ± 7.71 g kg−1), indicating led to a general decrease of OC in sediments in our studied depletion of OC in sediment at this scale. This clearly con- catchment compared to soils, three of them closely inter- trasts with experimental data from erosion plots that usually act: (i) sources of sediment linked to specific erosion pro- show higher concentrations of OC in sediments than in the cesses, (ii) size selectivity, and (iii) processes affecting the original source soils (Owens et al., 2002; Girmay et al., 2009; organic components of sediments during the erosion and flu- www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 1218 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics

Table 2. Percentages of microaggregated particles in different size classes and microaggregation indices (IA, Wang et al., 2010; ASC, Igwe, 2000).

Group 250–63 µm 63–20 µm 20–2 µm IA ASC name Soils 24.99 ± 16.27aA∗ 3.91 ± 6.99aB 7.34 ± 16.06aB 73.91 ± 21.03a 12.51 ± 34.91a (N = 16) Wedges 36.49 ± 17.88aA 9.59 ± 8.87bB 0 92.18 ± 23.50a 36.48 ± 17.92a (N = 25) Suspended 3.60 ± 9.21bA 3.19 ± 3.22aA 2.52 ± 6.40aA 18.22 ± 12.29b 0.98 ± 2.26b sediment (N = 41) Reservoir 6.65 ± 7.87bA 1.46 ± 2.70aA 2.44 ± 3.46aA 24.94 ± 14.30b −0.41 ± 9.71b (N = 12)

∗ Different lower case letters mean significant differences among sediments and soil groups. Different capital letters mean significant differences within sediments and soil groups, with regard to microaggregated class sizes (Kruskal–Wallis test, p < 0.05). vial transport pathway such as burial, mineralisation or new ers, mobilised by non-selective erosion processes such as OC formation. bank, gully and channel erosion, with lower OC concentrations than the surface soil. Geomorphological field assess- 5.2.1 Size selectivity and sources of sediment ments in the area indicated that these deeper operating erosion processes are indeed important sources of sediments in Wang et al. (2010) explained how selectivity of soil mate- the catchment (Boix-Fayos et al., 2007). Yet, sediments de- rial determines the OC concentration during transport. They rived from deep soil layers not only have lower OC concen- concluded that there was very little mineralisation of TOC tration but also different OC pool composition (Nadeu et al., during the erosion process, since they found higher C : N 2011) and different turnover rates (Nadeu et al., 2012) than ratios in sediments than in source soils and similar enrich- those derived from topsoil. Differences in OC pools between ment ratios of clay and C in deposited sediments. Based sediment deposits can also be the result of transport con- on multiple indicators our data show evidence for the conditions due to higher or lower rainfall intensity (Martínez- trary in the middle and lower catchment deposits (suspended Mena et al., 2012; Zhang et al., 2013) or to the discharge. sediments, delta and reservoir): higher clay contents, lower Smith et al. (2013) related high discharge rates with trans- OC and lower C : N ratios in sediments than in soils. These port of modern C, associated with erosion and the release of findings could indicate C losses by mineralisation during organic matter (Sanchez-Vidal et al., 2013), and C from vas- transport over longer distances. However, the interpretation cular sources (Goñi et al., 2013). Low flows were associated of narrowed C : N ratios as indicators of mineralisation must with export of fossil OC (Smith et al., 2013) from biogenic be done cautiously and in combination with other indicators sources dominated by non-vascular plants (Goñi et al., 2013) (Wang et al., 2010), given the complexity of N behaviour or from freshwater primary producers (Sanchez-Vidal et al., along the fluvial path (Robertson and Groffman, 2007). This 2013). is even more complex in sediments of intermittent rivers A comparison of the particle sizes of the studied sediments (as in our case) because nitrification and denitrification pro- and soils of the catchment points to a selection of trans- cesses are also dependent on the drying and rewetting cycles ported material in a downstream direction. Finer microaggre- of sediments (Gómez et al., 2012; Arce et al., 2013). In our gated and single particles were present in the suspended load study, total OC and N were lower in all sediment deposits and in the reservoir. In the soils and in the alluvial wedges compared to soils, with the exception of suspended load. In- behind check dams, the material was much more heteroge- crease of N in sediments rich in clay (as suspended load) has neous, having similar particle size distribution and larger mi- been attributed to the presence of ammonium or to the con- croaggregates, indicating again its transport by non-selective tribution of freshwater algae (Sanchez-Vidal et al., 2013). erosion processes and over short distances. The low OC concentration of sediments deposited in the upper catchment may have a different explanation. The 5.2.2 Processes affecting organic carbon dynamics sediments in the alluvial wedges showed textural classes and during transport and deposition C : N ratios similar to those of the soils, but lower OC concentrations. The low OC concentration may be explained by The OC concentration, the variation in C : N ratios and the the fact that the sediments originate from deeper soil lay- different associations of OC with textural classes in the

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1219

Table 3. Spearman correlation coefﬁcients between primary and microaggregated particle indicators and organic carbon pools and C : N ratios.

Microaggregated particles Primary particles Groups 250–63 µm 63–20 µm 20–2 µm IA ASC Sand Silt Clay TOC All data 0.36a – 0.32 – 0.70 −0.26 −0.64 Soils – – – – −0.51 0.57 – – Wedges – 0.36 –– −0.49 – 0.49 Suspended sed – – 0.34b –– −0.48 0.47 Reservoir – – 0.66 – 0.63 – −0.71 POC All data 0.58 – 0.51 0.58 ––– −0.27 Soils – – – – −0.61 – – – Wedges 0.52 – – 0.650 0.517 −0.72 0.45 – Suspended sed – – – – – – – – Reservoir – – – −0.67 0.82 – −0.87 MOC All data – 0.46 –––––– Soils – – – – – 0.58 – – Wedges – 0.57 –––– 0.72 Suspended sed – – – – – – – – Reservoir – – 0.66 – 0.62 – −0.72 C:N All data 0.49 0.333 0.27 −0.55 0.41 −0.26 −0.22 Soils 0.51 −0.70 ––– Wedges −0.55 0.75 – −0.55 ––– Suspended sed −0.40 – 0.45 – – −0.25 – Reservoir – – – – – – – –

a Only significant correlations are shown. Bold correlation indices are for p < 0.005, the rest have p < 0.05. b This correlation has p < 0.06. studied deposits indicate different processes of loss or gain found in the POC : MOC ratios (Fig. 5a and b) which sup- of OC during the transport along the studied catchment. port our statement that the degree of OC mineralisation is The C : N ratios showed clearly two groups (Fig. 5a): increased during transport along the flow path (prevalence of lower values in suspended sediments, delta and reservoir more recalcitrant fractions downstream). sediments, and higher values in soils and alluvial wedges. Apart from the indications of OC mineralisation, the This suggests a relatively low mineralisation of OC in the MOC concentration of suspended sediments was low and sediments of alluvial wedges due to efficient burial or a very variable (3.8–0.1 %), which can be attributed to the short transport time, as reported also by Smith et al. (2013) diverse characteristics of the events that mobilise material and suggested by Ran et al. (2014). Further downstream from different sources (Smith et al., 2013; Goñi et al., 2013; in the suspended sediments, delta and reservoir sediments, Sanchez-Vidal et al., 2013) or to sediments mobilised by dif- the lower C : N ratios could indicate mineralisation of OC ferent erosion processes (Nadeu et al., 2011, 2012). in sediment transported over longer distances (Fig. 5a) Furthermore, the relatively high OC content of the reser- (Bouchez et al., 2010; Hovius et al., 2011; Raymond and voir sediments could indicate in situ organic matter forma- Bauer, 2001). Ran et al. (2014) recently reported a miner- tion from ecological lake processes stimulating primary pro- alisation of 27 % during erosion and transport of sediment duction (Einsele et al., 2001) or allochthonous OC input from and associated OC through the Yellow River catchment. Al- the establishment of vegetation and soil formation in the though, as previously stated, interpretation of C : N ratios for frequently exposed upper sediment layers. Moreover, while mineralisation must be done with caution. Nevertheless, and other deposits (wedges and suspended sediment) showed the interestingly, the observed trend in the C : N ratios in soil and well-known relationship between clay and OC (Rodriguez- among different sediment deposits was consistent with that Rodriguez, 2004; Rhoton et al., 2006; Martínez-Mena et www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 1220 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics al., 2008), reservoir sediments presented a correlation be- of OC along the flow path, the OC being protected more tween the sand fraction and concentration of OC in the two when associated with large microaggregated particles in pools that could be indicative of in situ C formation. Au- soils and in the deposits of the upper catchment areas. tochthonous input of organic matter in rivers and lakes can account for around 50 % of the total organic matter in aquatic 5.3 Implications for the fate of eroded OC ecosystems of tropical-semiarid and dryland areas (Kunz et al., 2011; Medeiros and Arthington, 2011). The fluctua- The results from this study suggest that sediment reaching tion in autochthonous organic matter production in riverine depositional settings is composed of a heterogeneous mix- ecosystems depends on the input from terrestrial land uses in ture of OC particles in different stages of decomposition. the drainage area, with thresholds in which terrestrially de- The role of the source area, sediment transport and post- rived C is replaced by in-stream algal productivity (Hagen deposition processes were revealed as crucial to understand- et al., 2010). However, it can also be influenced by river hy- ing the characteristics of the OC and differences among the drodynamics (Cabezas and Comín, 2010; Devesa-Rey and analysed deposits and distinct phases of the erosion process. Barral, 2012). In particular, in Mediterranean river ecosys- Although OC mineralisation fluxes were not addressed di- tems there is an important seasonal shift between inputs of rectly, the decrease in the level of particle aggregation down- allochthonous and autochthonous organic matter, with high stream suggests a potential increase in OC decomposition primary production in spring and allochthonous organic mat- by microorganisms, leading to higher potential mineralisa- ter inputs in autumn (Romaní et al., 2013). Given that C : N tion rates. Distance from source areas, selective transport and ratios of allochthonous organic matter (more recalcitrant and deposition of sediments were identified as important factors resistant to degradation by microorganisms) tend to be higher controlling the characteristics of the OC in sediments and its than in situ-produced organic matter (Devesa-Rey and Bar- fate. ral, 2012), shifts between one and the other will change OC decomposition rates and dynamics. 6 Conclusions The combined interpretation of OC pools, textural analysis and C : N ratios in the soils and different sediment deposits A non-homogeneous redistribution of OC by water flow indicates that catchment-scale C redistribution by lateral takes place within catchments, which can be associated with fluxes is controlled by both the organic and the mineral na- the geomorphological processes and dynamics of sediment ture of sediments: transport and deposition. The redistribution of OC in sediments at the catchment scale is controlled by factors affecting 1. Sediments in upstream depositional areas (alluvial their organic component (mineralisation, protection of OC wedges) showed significantly lower C concentrations within microaggregates and new OC formation in some de- than soils, but sediment texture and C characteristics posits) and by factors affecting their mineral component (se- were more similar to those in soils than to those in lectivity of sediment sizes during the detachment, transport sediments transported further downstream, showing lit- and deposition phases of erosion, and the type of erosion pro- tle indication of mineralisation (similar C : N than in cesses: selective versus non-selective). soils) and low selectivity of particles (similar primary The processes that determine OC concentration at differ- and aggregated particle size distribution to soils). This is ent pools are related also to the different phases of erosion probably related to the non-selective character of domi- (detachment, transport and deposition): (i) during detach- nant erosion processes and to the proximity of sediment ment – size selectivity, type of erosion process and source of sources, giving little time for aggregate breakdown or C material; (ii) during transport – size selectivity, protection of mineralisation. OC in microaggregates and transport distances, and (iii) dur- 2. In middle stream areas, preferential deposition of coarse ing deposition and in the post-deposition phase – size selec- particles can be seen in the channel bars, enriched in the tivity, protection of OC from mineralisation by stabilisation sand fraction and showing the lowest concentrations of of microaggregates and burial and new OC formation are im- all C fractions, among all deposits. portant. The OC mobilised in catchments is associated very closely 3. Downstream, the suspended sediment in transit and with the sediment dynamics and can have long residence the sedimentary deposits (delta and reservoir) showed times linked to the fate of the sediments. In addition, it can higher contents of fine particles (clay and silt), accom- be increased by ecological processes and by replacement in panied by lower C : N ratios and a slightly higher C eroded areas, converting catchments into relevant sinks for C concentration, though still lower than in soils. The sand budgets. contents of the delta and reservoir deposits indicate also bedload contribution to the deposits downstream. The differences in the C : N ratio combined with other indicators could point to different degrees of mineralisation

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1221

Acknowledgements. This work has been financially supported Implications for their ecological restoration, Ecol. Eng., 36, 640– by the project ESUMA (11859/PI/09) of the Seneca Foundation, 652, 2010. Regional Government of Murcia (Spain) and by the project Cambardella, C. A. and Elliot, E. T.: Particulate soil organic-matter SOGLO (P7/24 IAP BELSPO) from the Belgian Government. changes across a grassland cultivation sequence, Soil Sci. Soc. Joris de Vente was supported by a Juan de la Cierva contract Am. J., 56, 777–783, 1992. (JCI-2011-08941). Elisabet Nadeu acknowledges financial support Chaplot, V. and Poesen, J.: Sediment, soil organic carbon and runoff from an FPI predoctoral fellowship of the former Ministry of delivery at various spatial scales, Catena, 88, 46–56, 2012. Science and Innovation (BES-2008-002379). We also thank the Devesa-Rey, R. and Barral, M. T.: Allochthonous versus au- MIRAGE project (EC, FP7-ENV-2007-1) and Gonzalo G. Barberá tochthonous naturally occurring organic matter in the Anllóns for facilitating the installation of the suspended solids sampling river bed sediments (Spain), Environ. Earth Sci., 66, 773–782, devices. Gonzalo G. Barberá advised also about the statistical 2012. approach. Rosa Gómez Cerezo, María del Mar Sánchez Montoya Einsele, G., Yan, J., and Hinderer, M.: Atmospheric carbon burial and Marisa Arce are thanked for providing a part of the samples in modern lake basins and its significance for the global carbon of the fluvial bars group. Many thanks to Pedro Pérez Cutillas for budget, Global Planet. Change, 30, 167–195, 2001. producing Fig. 1. Evans, W., Hales, B., and Strutton, P. G.: P CO2 distributions and air-water CO2 fluxes in the Columbia River estuary, Estuar. Edited by: M. Gooseff Coast. Shelf Sci., 117, 260–272, 2013. Fiener, P., Auerswald, K., and Weigand, S.: Managing erosion and water quality in agricultural watersheds by small detention ponds, Agr. Ecosyst. Environ., 110, 132–142, 2005. Girmay, G., Singh, B. R., Nyssen, J., and Borrosen, T.: Runoff and References sediment-associated nutrient losses under different land uses in Tigray, Northern Ethiopia, J. Hydrol., 376, 70–80, 2009. Amegashie, B. K., Quansah, C., Agyare, W. A., Tamene, L., and Gómez, R., Arce, I. M., Sánchez, J. J., and Sánchez-Montoya, M. Vlek, P. L. G.: Sediment-bound nutrient export from five small M.: The effects of drying on sediment nitrogen content in a reservoir catchments and its implications for the Sudan savanna Mediterranean intermittent stream: A microcosms study, Hydro- zone of Ghana, Lakes Reserv. Res. Manage., 16, 61–76, 2011. biologia, 679, 43–59, 2012. Arce, M. I., Gómez, R., Suárez, M. L., and Vidal-Abarca, M. R.: Goñi, M. A., Hatten, J. A., Wheatcroft, R. A., and Borgeld, J. C.: Denitrification rates and controlling factors in two agriculturally Particulate organic matter export by two contrasting small moun- influenced temporary Mediterranean saline streams, Hydrobiolo- tainous rivers from the Pacific Northwest, U.S.A., J. Geophys. gia, 700, 169–185, 2013. Res.-Biogeo., 118, 112–134, 2013. Aufdenkampe, A. K., Mayorga, E., Raymond, P. A., Melack, J. M., Hagen, E. M., McTammany, M. E., Webster, J. R., and Benfield, E. Doney, S. C., Alin, S. R., Aalto, R. E., and Yoo, K.: Riverine F.: Shifts in allochthonous input and autochthonous production in coupling of biogeochemical cycles between land, oceans, and at- streams along an agricultural land-use gradient, Hydrobiologia, mosphere, Front. Ecol. Environ., 9, 53–60, 2011. 655, 61–77, 2010. Avnimelech, Y. and McHenry, J. R.: Enrichment of transported sed- Harden, J. W., Sharpe, J. M., Parton, W. J., Ojima, D. S., Fries, T. iments with organic carbon, nutrients and clay, Soil Sci. Soc. Am. L., Huntington, T. G., and Dabney, S. M.: Dynamic replacement J., 48, 259–266, 1984. and loss of soil carbon on eroding cropland, Global Biogeochem. Berhe, A. A. and Kleber, M.: Erosion, deposition, and the per- Cy., 13, 885–901, 1999. sistence of soil organic matter: Mechanistic considerations and Haregeweyn, N., Poesen, J., Deckers, J., Nyssen, J., Haile, M., Gov- problems with terminology, Earth Surf. Proc. Land., 38, 908– ers, G., Verstraeten, G., and Moeyersons, J.: Sediment-bound nu- 912, 2013. trient export from micro-dam catchments in Northern Ethiopia, Boix-Fayos, C., Barbera, G. G., Lopez-Bermudez, F., and Castillo, Land Degrad. Develop., 19, 136–152, 2008. V. M.: Effects of check dams, reforestation and land use changes Hoffmann, T., Mudd, S. M., van Oost, K., Verstraeten, G., Erkens, on river channel morphology: Case study of the Rogativa catch- G., Lang, A., Middelkoop, H., Boyle, J., Kaplan, J. O., Willen- ment (Murcia, Spain), Geomorphology, 91, 103–123, 2007. bring, J., and Aalto, R.: Short Communication: Humans and the Boix-Fayos, C., de Vente, J., Martínez-Mena, M., Barberá, G. G., missing C-sink: erosion and burial of soil carbon through time, and Castillo, V.: The impact of land use change and check-dams Earth Surf. Dynam. Discuss., 1, 93–112, 2013. on catchment sediment yield, Hydrol. Process., 22, 4922–4935, Hovius, N., Galy, A., Hilton, R. G., Sparkes, R., Smith, J., Shuh-Ji, 2008. K., Hongey, C., In-Tian, L., and Joshua West, A.: Erosion-driven Boix-Fayos, C., de Vente, J., Albaladejo, J., and Martínez-Mena, drawdown of atmospheric carbon dioxide: The organic pathway, M.: Soil carbon erosion and stock as affected by land use Appl.Geochem., 26, S285–S287, 2011. changes at the catchment scale in Mediterranean ecosystems, IGME: Mapa Geológico de España 1 : 50 000, Nerpio, 909, Instituto Agr. Ecosyst. Environ., 133, 75–85, 2009. Geológico y Minero de España, 1978. Bouchez, J., Beyssac, O., Galy, V., Gaillardet, J., France-Lanord, Igwe, C. A.: Nutrient losses in runoff and eroded sediments from C., Maurice, L., and Moreira-Turcq, P.: Oxidation of petrogenic soils of central eastern Nigeria, Polish J. Soil Sci., 33, 67–75, organic carbon in the Amazon floodplain as a source of atmo- 2000. spheric CO2, Geology, 38, 255–258, 2010. IUSS Working Group WRB: World reference base for soil re- Cabezas, A. and Comín, F. A.: Carbon and nitrogen accretion in sources, Word Soil Resources Report 103, FAO, Rome, 2006. the topsoil of the Middle Ebro River Floodplains (NE Spain): www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 1222 C. Boix-Fayos et al.: Sediment flow paths and associated organic carbon dynamics

Kunz, M. J., Anselmetti, F. S., West, A., Wehrli, B., Vollenweider, dams to reduce catchment sediment yield, Land Degrad. De- A., Thüring, S., and Senn, D. B.: Sediment accumulation and velop., doi:10.1002/ldr.2331, in press, 2014. carbon, nitrogen, and phosphorus deposition in the large tropical Ran, L., Lu, X. X., and Xin, Z.: Erosion-induced massive or- reservoir Lake Kariba (Zambia/Zimbabwe), J. Geophys. Res.- ganic carbon burial and carbon emission in the Yellow River Biogeo., 116, G03003, doi:10.1029/2010JG001538, 2011. basin, China, Biogeosciences, 11, 945–959, doi:10.5194/bg-11- Le Quéré, C., Raupach, M. R., Canadell, J. G., Marland, G., Bopp, 945-2014, 2014. L., Ciais, P., Conway, T. J., Doney, S. C., Feely, R. A., Foster, Raymond, P. A. and Bauer, J. E.: Riverine export of aged terrestrial P., Friedlingstein, P., Gurney, K., Houghton, R. A., House, J. I., organic matter to the North Atlantic Ocean, Nature, 409, 497– Huntingford, C., Levy, P. E., Lomas, M. R., Majkut, J., Metzl, 500, 2001. N., Ometto, J. P., Peters, G. P., Prentice, I. C., Randerson, J. T., Rhoton, F. E., Emmerich, W. E., Goodrich, D. C., Miller, S. N., Running, S. W., Sarmiento, J. L., Schuster, U., Sitch, S., Taka- and McChesney, D. S.: Soil geomorphological characteristics of hashi, T., Viovy, N., Van Der Werf, G. R., and Woodward, F. I.:. a semiarid watershed: Influence on carbon distribution and trans- Trends in the sources and sinks of carbon dioxide, Nat. Geosci., port, Soil Sci. Soc. Am. J., 70, 1532–1540, 2006. 2, 831–836, 2009. Robertson, G. P. and Groffman, P. M.: Nitrogen transformations, Martínez-Mena, M., Lopez, J., Almagro, M., Boix-Fayos, C., and in: Soil Microbiology, Biogeochemistry, and Ecology, edited by: Albaladejo, J.: Effect of water erosion and cultivation on the soil Paul, E. A., Springer, New York, NY, USA, 341–364, 2007. carbon stock in a semiarid area of South-East Spain, Soil Till. Rodriguez-Rodriguez, A., Guerra, A., Arbelo, C., Mora, J. L., Gor- Res., 99, 119–129, 2008. rín, S. P., and Armas, C.: Forms of eroded soil organic carbon Martínez-Mena, M., López, J., Almagro, M., Albaladejo, J., in andosols of the Canary Islands (Spain), Geoderma, 121, 205– Castillo, V., Ortiz, R., and Boix-Fayos, C.: Organic carbon en- 219, 2004. richment in sediments: Effects of rainfall characteristics under Romaní, A. M., Amalfitano, S., Artigas, J., Fazi, S., Sabater, S., Ti- different land uses in a Mediterranean area, Catena, 94, 36–42, moner, X., Ylla, I., and Zoppini, A.: Microbial biofilm structure 2012. and organic matter use in mediterranean streams, Hydrobiologia, Meade, R. H. and Stevens Jr., H. H.: Strategies and equipment for 719, 43–58, 2013. sampling suspended sediment and associated toxic chemicals in Sanchez-Vidal, A., Higueras, M., Martí, E., Liquete, C., Calafat, large rivers – with emphasis on the Mississippi River, Sci. Total A., Kerhervé, P., and Canals, M.: Riverine transport of terrestrial Environ., 97–98, 125–135, 1990. organic matter to the North Catalan margin, NW Mediterranean Medeiros, E. S. F. and Arthington, A. H.: Allochthonous and au- Sea, Prog. Oceanogr., 118, 71–80, 2013. tochthonous carbon sources for fish in floodplain lagoons of an Smith, J. C., Galy, A., Hovius, N., Tye, A. M., Turowski, J. M., and Australian dryland river, Environ. Biol. Fish., 90, 1–17, 2011. Schleppi, P.: Runoff-driven export of particulate organic carbon Nadeu, E., de Vente, J., Martinez-Mena, M., and Boix-Fayos, C.: from soil in temperate forested uplands, Earth Planet. Sc. Lett., Exploring particle size distribution and organic carbon pools mo- 365, 198–208, 2013. bilized by different erosion processes at the catchment scale, J. Stallard, R. F.: Terrestrial sedimentation and the carbon cycle: Soils Sediments, 11, 667–678, 2011. coupling weathering and erosion to carbon burial, Global Bio- Nadeu, E., Berhe, A. A., de Vente, J., and Boix-Fayos, C.: Ero- geochem. Cy., 12, 231–257, 1998. sion, deposition and replacement of soil organic carbon in Tranvik, L. J., Downing, J. A., Cotner, J. B., Loiselle, S. A., Striegl, Mediterranean catchments: a geomorphological, isotopic and R. G., Ballatore, T. J., Dillon, P., Finlay, K., Fortino, K., Knoll, L. land use change approach, Biogeosciences, 9, 1099–1111, B., Kortelainen, P. L., Kutser, T., Larsen, S., Laurion, I., Leech, doi:10.5194/bg-9-1099-2012, 2012. D. M., Leigh McCallister, S., McKnight, D. M., Melack, J. M., Nadeu, E., Van Oost, K., Boix-Fayos, C., and de Vente, J.: Impor- Overholt, E., Porter, J. A., Prairie, Y.,Renwick, W. H., Roland, F., tance of land use patterns for erosion-induced carbon fluxes in a Sherman, B. S., Schindler, D. W., Sobek, S., Tremblay, A., Vanni, Mediterranean catchment, Agr. Ecosyst. Environ., 189, 181–189, M. J., Verschoor, A. M., Von Wachenfeldt, E., and Weyhenmeyer, 2014. G. A.: Lakes and reservoirs as regulators of carbon cycling and Nadeu, E., Quiñonero-Rubio, J. M., de Vente, J., and Boix-Fayos, climate, Limnol. Oceanogr., 54, 2298–2314, 2009. C.: The influence of catchment morphology, lithology and land Trimble, S. W.: A sediment budget for Coon Creek basin in the use on soil organic carbon export in a Mediterranean mountain Driftless Area, Wisconsin, 1853–1977, Am. J. Sci., 283, 454– region, Catena, 126, 117–125, doi:10.1016/j.catena.2014.11.006, 474, 1983. 2015. Van Hemelryck, H., Govers, G., Van Oost, K.. and Merckx, R.: Owens, L. B., Malone, R. W., Hothem, D. L., Starr, G. C. and Lal, Evaluating the impact of soil redistribution on the in situ min- R.: Sediment carbon concentration and transport from small wa- eralization of soil organic carbon, Earth Surf. Proc. Land., 36, tersheds under various conservation tillage practices, Soil Till. 427–438, 2011. Res., 67, 65–73, 2002. Van Oost, K., Quine, T. A., Govers, G., De Gryze, S., Six, J., Quiñonero-Rubio, J. M., Boix-Fayos, C., and de Vente, J.: Devel- Harden, J. W., Ritchie, J. C., McCarty, G. W., Heckrath, G., Kos- opment and application of a multi-factorial sediment connectiv- mas, C., Giraldez, J. V., da Silva, J. R. M., and Merckx, R.: The ity index at the catchment scale, Desarrollo y aplicación de un impact of agricultural soil erosion on the global carbon cycle, índice multifactorial de conectividad de sedimentos a escala de Science, 318, 626-629, 2007. cuenca, 39, 203–223, 2013. Van Oost, K., Verstraeten, G., Doetterl, S., Notebaert, B., Wiaux, F., Quiñonero-Rubio, J. M., Nadeu, E., Boix-Fayos, C., and de Vente, Broothaerts, N., and Six, J.: Legacy of human-induced C erosion J.: Evaluation of the effectiveness of forest restoration and check-

Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015 www.hydrol-earth-syst-sci.net/19/1209/2015/ C. Boix-Fayos et al.: Sediment ﬂow paths and associated organic carbon dynamics 1223

and burial on soil-atmosphere C exchange, P. Natl. Acad. Sci. Wisser, D., Frolking, S., Hagen, S., and Bierkens, M. F. P.: Beyond USA, 109, 19492–19497, 2012. peak reservoir storage? A global estimate of declining water stor- Verstraeten, G., Bazzofﬁ, P., Lajczak, A., Rãdoane, M., Rey, F., Poe- age capacity in large reservoirs, Water Resour. Res., 49, 5732– sen, J., and De Vente, J.: Reservoir and Pond Sedimentation in 5739, 2013. Europe, John Wiley & Sons, Chichester, UK, 757–774, 2006. Zhang, X., Li, Z., Tang, Z., Zeng, G., Huang, J., Guo, W., Chen, X., Walling, D. E. and Fang, D.: Recent trends in the suspended sedi- and Hirsh, A.: Effects of water erosion on the redistribution of ment loads o the world’s rivers, Global Planet. Change, 39, 111– soil organic carbon in the hilly red soil region of southern China, 126, 2003. Geomorphology, 197, 137–144, 2013. Wang, Z., Govers, G., Steegen, A., Clymans, W., Van den Putte, A., Langhans, C., Merckx, R., and Van Oost, K.: Catchment-scale carbon redistribution and delivery by water erosion in an inten- sively cultivated area, Geomorphology, 124, 65–74, 2010.

www.hydrol-earth-syst-sci.net/19/1209/2015/ Hydrol. Earth Syst. Sci., 19, 1209–1223, 2015